The present invention relates to a method for compensating for combustion chamber-specific actual values of a parameter, which characterizes combustions, between a plurality of combustion chambers of an Otto engine operated with homogeneous filling of the combustion chambers and homogeneous charge compression ignition. Said Otto engine operates with a direct fuel injection and a variable valve drive, wherein the combustion chamber-specific actual values are formed as a function of signals of combustion chamber-specific sensors and wherein deviations of the combustion chamber-specific actual values from a set value are processed to manipulated variables, with which a valve lift of a gas exchange valve of a combustion chamber is changed. The invention furthermore relates to a control unit equipped to carry out said method, a correspondingly programmed computer program as well as a computer program product including a computer program of this type.
Such subject matters are in each case known from the German patent publication DE 10 2004 032 986 A1.
New combustion processes for Otto engines, which in the literature are designated as HCCI (Homogeneous Charge Compression Ignition) or CAI method (Controlled Auto Ignition), distinguish themselves from the conventional spark-ignition operation by a significant potential for fuel savings. These fuel savings occur particularly in partial load ranges (relevant to a test cycle).
Moreover, additional advantages result with this type of combustion like very low raw emissions of pollutants, particularly in comparison with the likewise fuel saving stratified charge mode. Additional, relatively expensive exhaust gas aftertreatment systems such as NOx storage catalytic converters can therefore be dispensed with. These advantages are the result of the combination of different effects such as low combustion temperatures and a very homogeneous mixture formation, which lead to a plurality of exothermic centers in the combustion chamber and thereby to a very even and quickly completed combustion.
CAI engines are typically equipped with a variable valve drive and direct gasoline injection. Different strategies exist for producing the CAI operation, which all have the goal of providing a relatively large proportion of residual gas to the combustion chamber contents of the Otto engine. The temperature of the combustion chamber content is increased by the hot residual gas so that the autoignition temperature is achieved during the compression phase and combustion is initiated without an additional triggering event being required like an ignition spark in conventional combustion processes of Otto engines or an injection of fuel as in combustion processes of diesel engines.
The large proportion of residual gas is, for example, realized as a result of the internal combustion engine being operated without valve overlap of the intake valves and exhaust valves of its combustion chambers, which also is described as negative valve overlap. A certain quantity of residual gas is thereby retained in the cylinder, which experiences an intermediate compression. As an alternative, said residual gas can be externally recirculated or can be drawn back into the combustion chamber by a short-term opening of the exhaust valve during the intake phase.
The optimized open-loop control, respectively closed-loop control, of the combustion on the basis of a combustion chamber signal plays a decisive role in the case of CAI combustion processes.
In the method known from the German patent publication DE 10 2004 032 986 A1, the position of a 50% mass conversion point is used as the parameter characterizing combustions. This point is characterized by the fact that 50% of the fuel proportion of a combustion chamber content is thereby combusted. In so doing, the determination of the 50% mass conversion point occurs collectively for all combustion chambers from the signal of a lambda sensor disposed in the total exhaust of the combustion chambers and/or in a combustion chamber-specific manner from the signals of cylinder-specific pressure sensors or ion current sensors.
In the known method, the 50% mass conversion point is adjusted in a first control circuit to a set value, which is not combustion chamber-specific. In a second control circuit, the parameters acquired in a combustion chamber-specific manner are additionally adjusted to each other.
In the German patent publication DE 10 2004 032 986, the use of the second control circuit is based on the grounds that an actual combustion behavior that deviates from the optimal combustion behavior can still occur in individual cylinders when the first control circuit is in a steady-state operation. This can then lead to results which have been negatively influenced. Different states of wear of the cylinders are seen to be a possible cause of such deviations.
All in all a mean value of the parameter, which is influenced by all of the combustion chambers, is acquired in the first control circuit and is adjusted to a set value, which is the same for all combustion chambers, with a manipulated variable that affects all of said combustion chambers. In the second control circuit, parameters acquired in a combustion chamber-specific manner are adjusted by manipulated variables formed in a combustion chamber-specific manner.
The phase positions of the opening of exhaust valves of the combustion chambers are used as the manipulated variable for changing the position of the 50% mass conversion point in the first control circuit. Injection pulse widths of pre-injections or main injections of fuel into the combustion chambers are used as said manipulated variable in the second control circuit. The intervention on gas exchange valves mentioned at the beginning of the application therefore refers to intervention on exhaust valves.